Xmr sensors with reduced discontinuities

ABSTRACT

Embodiments relate to xMR sensors, including giant magnetoresistive (GMR), tunneling magnetoresistive (TMR) or anisotropic magnetoresistive (AMR), and the configuration of xMR strips within xMR sensors. In an embodiment, an xMR strip includes a plurality of differently sized and/or differently oriented serially connected portions. In another embodiment, an xMR strip includes a varying width or other characteristic. Such configurations can address discontinuities associated with conventional xMR sensors and improve xMR sensor performance.

TECHNICAL FIELD

The invention relates generally to integrated circuit (IC) sensors andmore particularly to magnetoresistive IC sensors for rotational speedsensing applications.

BACKGROUND

Magnetoresistive sensors can include giant magnetoresistive (GMR),tunnel magnetoresistive (TMR), anisotropic magnetoresistive (AMR) andother technologies, referred to collectively as xMR technologies. XMRtechnologies can be used in rotational speed sensors, whichconventionally include a pole wheel transducer wheel or ferrous wheel incombination with a magnetic back bias field perpendicular to the sensorplane. The xMR structures positioned near the wheel in these sensors aretypically narrow strips with a homogenous width of, for example, lessthan 2 micrometers (μm) in order to provide a defined sensitivity by theshape anisotropy and a length of about, for example, 300 μm.

Transducer pole wheels have only a limited thickness, or axial width,such that the magnetic signal field is not homogenous over the entirexMR strip length. As axial distance increases from the pole wheelcenter, the By component increases and is phase-shifted by 180 degreesbetween the lower and upper half of the pole wheel. In combination witha Bx component which is phase-shifted by +/−90 degrees to the individualBy components, the result is a rotation of the magnetic field vector inopposite senses of rotation for the lower and upper halves of the polewheel and, as a consequence, for the lower and upper half of the xMRstrip in a centered adjustment regarding the pole wheel. Depending onthe pole wheel width and ratio/magnitude of the Bx/By components as afunction of axial position, the counterrotating magnetic fields can alsoinduce a counterrotating magnetization of the sensor layer, therebygenerating temporary domain walls. The generation and annihilation ofthe domain walls results in a discontinuity, or jitter, in the outputsignal which can disturb the interpretation of the signal.

The shape anisotropy of the xMR strip can also result in a differentbehavior of the magnetization over the strip width. Whereas the domainsin the center can follow an external magnetic field quite easily, themagnetization regions at the edges are much more stable. Domain wallsbetween the strip center and edge can occur, and magnetizationdirections in the edge regions can suddenly turn along the externalfield when the field angle or field strength exceeds a certain value.This switching process is reflected in a discontinuity in the resistancecharacteristic. The field angle where the switching occurs, for a givensensor material (e.g., free layer thickness material, etc.), depends onthe field conditions (By/Bx ratio) of the rotating field vector and theshape anisotropy and, therefore, the stripe width. The discontinuity inthe output signal can affect the interpretation of the signal viajitter, pulse loss, etc.

Conventional approaches to addressing these problems include mounting aback bias magnet on a rear side of an xMR sensor to create a biasmagnetic field to stabilize the direction of the free layermagnetization which can prevent free layer rotation but requiressignificant effort and can lead to a reduction in sensor sensitivity.Therefore, new solutions to minimize discontinuities in the outputsignal of xMR speed sensors are desired.

SUMMARY

In an embodiment, a magnetoresistive sensor element comprises amagnetoresistive strip comprising a plurality of serial segments,adjacent ones of the segments having different tilt angles.

In another embodiment, a magnetoresistive sensor element comprises amagnetoresistive strip comprising at least two segments seriallycoupled, a first segment having a negative curve and a second segmenthaving a positive curve.

In another embodiment, a magnetoresistive sensor element comprises amagnetoresistive strip comprising a plurality of serially coupledsegments, adjacent ones of the plurality of serially coupled segmentshaving different widths.

In another embodiment, a method comprises forming a magnetoresistivestrip having a first end and a second end; and adjusting a width of thefirst and second ends relative to a width of a center of themagnetoresistive strip to compensate for an unwanted magnetic fieldcomponent.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 depicts xMR strip tilt angles according to an embodiment.

FIG. 2 depicts simulation results for various tilt angles according toan embodiment.

FIG. 3 depicts simulation results for a mean tilt angle according to anembodiment.

FIG. 4 depicts an xMR strip according to an embodiment.

FIG. 5 depicts an xMR strip according to an embodiment.

FIG. 6A depicts an xMR strip according to an embodiment.

FIG. 6B depicts an xMR strip according to an embodiment.

FIG. 6C depicts an xMR strip according to an embodiment.

FIG. 6D depicts an xMR strip according to an embodiment.

FIG. 6E depicts an xMR strip according to an embodiment.

FIG. 7 depicts an xMR strip according to an embodiment.

FIG. 8 depicts an xMR strip according to an embodiment.

FIG. 9 depicts simulation results for various widths according to anembodiment.

FIG. 10 depicts an xMR strip according to an embodiment.

FIG. 11 depicts an xMR strip according to an embodiment.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

Embodiments relate to xMR sensors, including giant magnetoresistive(GMR), tunneling magnetoresistive (TMR) or anisotropic magnetoresistive(AMR), and the configuration of xMR strips within xMR sensors. GMR andTMR structure can be of the so-called spin-valve type in embodiments.Basically, a spin-valve comprises two functional magnetic layers: afirst ferromagnetic layer (e.g., CoFe, NiFe, etc.) is magneticallypinned to a natural antiferromagnet (eg., PtMn, IrMn, etc.), forming afixed reference magnetization, and a neighboring second ferromagneticlayer is magnetically decoupled by a non-magnetic interlayer (e.g., Cu,etc.) and forms a so-called free layer which can ideally follow thedirection of an external in-plane magnetic field. Often, the singlepinned layer is replaced by an artificial antiferromagnet having apinned layer coupled to a natural antiferromagnet and a so-calledreference layer which is antiferromagnetically coupled to the pinnedlayer by the RKKY exchange. This construction is more stable withrespect to high magnetic field strengths.

In an embodiment, an xMR strip comprises a plurality of differentlysized and/or differently oriented serially connected portions. Inanother embodiment, an xMR strip comprises a varying width or othercharacteristic. Such configurations can address discontinuitiesassociated with conventional xMR sensors and improve xMR sensorperformance.

In an xMR sensor, the magnetic field angle where edge magnetizationswitching occurs can depend on an angle between the structureorientation (shape anisotropy axis) and the external magnetic field. ForGMR and TMR structures, the orientation of the reference magnetizationdetermines at which signal level the discontinuity occurs. In anembodiment, a plurality of serially connected elements with differentorientations (i.e., tilt angles) with respect to the referencemagnetization form an xMR strip. In a rotating field, a discontinuitywill occur at different field angles for the varying strip elementorientations, and the more pronounced discontinuities of conventionalsensors are reduced by averaging over a number of phase-shifteddiscontinuities.

FIG. 1 depicts positive and negative tilt angles of an xMR strip 100.XMR strip 100 is depicted with both negative and positive tilt angles,φ, with respect to an external field angle. The orientation of thepinned layer magnetization is constant.

FIG. 2 is a graph of simulated output signals for five different tiltangles, φ: 0 degrees, +/−20 degrees and +/−45 degrees. Discontinuitiesfor each angle are circled for convenience of illustration. As can beseen, discontinuities for the various tilt angles occur at differentmagnetic field angles. For example, a discontinuity for a tilt angle of−20 degrees occurs at a magnetic field angle of about 35 degrees, whilea discontinuity for a tilt angle of +20 degrees occurs at a magneticfield angle of about 75 degrees.

If the signals depicted in FIG. 2 are averaged according to anembodiment, however, a significantly smoother characteristic can beobtained. This is shown in FIG. 3, which also includes the simulationfor 0 degrees as depicted in FIG. 2 for comparison. As can be seen inFIG. 3, large discontinuities are no longer present in the mean signal,rather only minor ripples which are circled. In embodiments, theseripples can be removed through a further discretization of the meansignal. A discontinuity in the signal at 0 degrees can still be seen inFIG. 3.

Expanding on the results of FIG. 3, embodiments therefore form xMRstrips of a plurality of serially connected xMR portions, segments orelements having varying tilt angles and configurations with respect tothe reference magnetization axis. These varying tilt angles can providea “mean” output signal with reduced or eliminated discontinuities inembodiments.

An embodiment of an xMR strip 200 is depicted in FIG. 4. XMR strip 200comprises a plurality of xMR segments 200 a-200 n. The number of xMRsegments 200 a-200 n can vary in embodiments. In the embodiment of FIG.4, adjacent ones of the xMR segments 200 a-200 n have opposite tiltangles; while xMR segment 200 a has a negative tilt angle, xMR segment200 b has a positive tilt angle. The degree of tilt angle also varies,as can be seen between xMR segment 200 a when compared with xMR segment200 e. A central xMR segment 200 j has a tilt angle of 0 degrees in thisembodiment. In addition to have varying tilt angles, xMR segments 200a-200 n can have varying lengths; compare again, for example, segment200 a with segment 200 e. By varying the length of a particular element,its weight in the averaging function can be defined. The particularconfiguration of strip 200 is exemplary of only one embodiment, andother configurations having differently varying tilt angles and lengthsare contemplated in other embodiments.

Another embodiment of an xMR strip 300 is depicted in FIG. 5. XMR strip300 comprises a continuously increasing and decreasing tilt angleS-shape. The configuration of xMR strip 300 can be thought of as havingthree segments: a negatively curved segment 300 a, a 0-degree segment300 b and a positively curved segment 300 c. Because of the similar butopposite orientations of segments 300 a and 300 c, xMR strip 300provides equal portions of positively and negatively tilted orientationsto provide an effective averaging, as in other embodiments discussedabove. As in FIG. 4, the particular configuration of strip 300 isexemplary of only one embodiment, and other configurations havingdifferently curved segments, lengths and/or orientations arecontemplated in other embodiments.

Other xMR strip embodiments are depicted in FIGS. 6A and 6B. In FIGS. 6Aand 6B, XMR strip 400 can have a reduced height as compared with otherembodiments by dividing strip 400 into a plurality of serially (FIG. 6A)or parallel (FIG. 6B) coupled adjacent substrips 402, 404 and 406 bysuitable metal or other connectors 408. In embodiments, substrips 402,404 and 406 are arranged substantially parallel to one another, asdepicted in FIGS. 6A and 6B. FIG. 6B additionally includes substri 407.Each substrip 402, 404 and 406 comprises a plurality of xMR segmentshaving alternating tilt angles, as in other embodiments. In theembodiments of FIGS. 6A and 6B, the degree of tilt angle generallyvaries between substrips. For example, the tilt angles of segments ofsubstrip 402 are greater than those of segments of substrip 404, whichare in turn greater than those of segments of substrip 406. Such aconfiguration can still present an effective signal averaging even underinhomogeneous magnetic field conditions. In addition, the width of thesubstrips can also vary, thereby influencing the angle phase of theswitching discontinuity which can lead to an effective geometricaveraging (refer, for example to FIG. 6B). As in FIGS. 4 and 5, theparticular configuration of strip 400 is exemplary of only oneembodiment, and other configurations having differently varying tiltangles, lengths, numbers and configurations of substrips and/or segmentsand other characteristics are contemplated in other embodiments.

Another xMR strip 450 is depicted in FIG. 6C. XMR strip 450 comprisesserially coupled segments 452, 454, 456 and 458, each having a differentwidth and with the same length axis orientation. Because of the widthdependent shape anisotropy, each segment 452-458 exhibits a differentphase angle of the switching discontinuity and, therefore, an averagingeffect takes place. While the widths increase from segment 452 to 458,other configurations are possible in other embodiments, FIG. 6C beingexemplary of but one embodiment.

Another embodiment is depicted in FIG. 6D. In FIG. 6D, xMR strip 460comprises a plurality of serially coupled strip segments 462, 464, 466and 468, each having the same nominal width and length axis orientation.Because segments 462-468 do not have identical values for the shapeanisotropy, e.g. because of local fluctuations of the xMR structuringprocess, the phase angle of the switching discontinuity will differ forthe individual segments 462-468. As a result, an averaging effect takesplace. This also occurs if a long strip is subdivided into substripswhich are serially connected by connectors, thereby keeping the fullstrip length, as is depicted in strip 470 in FIG. 6E. The decoupling ofthe magnetic properties allows single substrips 472, 474, 476 to reactindividually according to natural fluctuations of their magneticproperties on the external magnetic field, which leads to an averagingeffect.

Another xMR strip 500 is depicted in FIG. 7, somewhat similarconceptually to FIG. 5. XMR strip 500 comprises two segments 500 a and500 b serially connected by a suitable metal or other connector 502. Asin xMR strip 300, strip 500 presents a continuous distribution of tiltangles, providing a highly effective geometric averaging of the magneticbehavior of segments 500 a and 500 b.

As previously mentioned, discontinuities can also be addressed inembodiments by an xMR strip comprising a varying width or othercharacteristic, such as a shape anisotropy. Conventional xMR sensorscomprise strips such as those depicted in FIG. 1, isotropic rectangles.Such strips, however, can generate 180 degree domain walls because thefree layer magnetization is exposed to rotating fields with opposingsenses of rotation due to parasitic By components. Embodiments discussedherein, however, avoid these domain walls and their relateddiscontinuities by preventing the magnetizations from fully rotating bya varying, adapted xMR strip width. A reduced strip width results in anincreased shape anisotropy therefore in an enhanced stabilization of themagnetization in regions in which a rotation of the magnetic field takesplace. The stronger the By component of a rotating field, the smallerthe strip width needs to be. As a consequence, the strip width iscontinuously reduced toward the strip ends in an embodiment.

An embodiment is depicted in FIG. 8. In FIG. 8, xMR strip 600 has afirst width w1 at each end and a second width w2 in the middle, with thewidth varying between each end and the middle. In another embodiment,strip 600 has different widths at each end.

Simulation results for xMR strip 600 are depicted in the graph of FIG.9, in which w1 is 1 micrometer (μm) and w2 is 1.5 μm. Also depicted aresimulation results for two conventional strips having widths of 1 μm and1.5 μm. As can be seen, no discontinuities or sudden jumps in the outputsignal are observed for the wedge-shaped strip 600 because no 180-degreedomain walls are generated. Further, for the conventional strip with awidth of 1 μm, no sudden jump in the output signal is seen, but theenhanced shape anisotropy leads to a disadvantageous reduction insensitivity.

Additional embodiments are depicted in FIGS. 10 and 11. In FIG. 10, anXMR strip 700 has a similar profile to that of strip 600 of FIG. 8, butstrip 700 comprises a plurality of individual strip segments 700 a-700 ecoupled by suitable metal or other connectors 702. While strip 700comprises five segments 700 a-700 e, other embodiments can comprise moreor fewer segments. Further, each individual segment can be an isotropicrectangle or have a varying width in various embodiments.

In FIG. 11, an xMR strip 800 has an inverse wedge shape in which w1 ateach end is greater than w2 in the middle. Strip 800 is similar to butthe opposite of strip 600 of FIG. 8. As for strip 600, strip 800 canalso have different widths at each end. Strip 800 can be used inembodiments when a backbias magnet with a magnetization perpendicular tothe sensor plane is used, comprising a symmetric, bipolar bias fieldalong the strip length (By component). The magnitude of the By componentincreases from the center to the strip ends. An increase of the stripwidth towards the ends leads to a gradually reduced shape anisotropywhich therefore has a compensating effect on the reduced sensitivity bythe By component. As a result, the sensitivity of the entire strip 800is not unnecessarily decreased.

Embodiments can also comprise hybrid configurations of various stripsdiscussed herein. For example, strip 200 can comprise segments 200 a-200n shaped like strip 600. Various other configurations are also possible,as appreciated by those having skill in the art, for example thecombination of different widths and angles.

Embodiments discussed herein can therefore address discontinuitiesassociated with conventional xMR sensors. Embodiments thus can improvexMR sensor performance and provide advantages over conventional sensors,including reduced or eliminated discontinuities.

Various embodiments of systems, devices and methods have been describedherein. These embodiments are given only by way of example and are notintended to limit the scope of the invention. It should be appreciated,moreover, that the various features of the embodiments that have beendescribed may be combined in various ways to produce numerous additionalembodiments. Moreover, while various materials, dimensions, shapes,implantation locations, etc. have been described for use with disclosedembodiments, others besides those disclosed may be utilized withoutexceeding the scope of the invention.

Persons of ordinary skill in the relevant arts will recognize that theinvention may comprise fewer features than illustrated in any individualembodiment described above. The embodiments described herein are notmeant to be an exhaustive presentation of the ways in which the variousfeatures of the invention may be combined. Accordingly, the embodimentsare not mutually exclusive combinations of features; rather, theinvention may comprise a combination of different individual featuresselected from different individual embodiments, as understood by personsof ordinary skill in the art.

Any incorporation by reference of documents above is limited such thatno subject matter is incorporated that is contrary to the explicitdisclosure herein. Any incorporation by reference of documents above isfurther limited such that no claims included in the documents areincorporated by reference herein. Any incorporation by reference ofdocuments above is yet further limited such that any definitionsprovided in the documents are not incorporated by reference hereinunless expressly included herein.

For purposes of interpreting the claims for the present invention, it isexpressly intended that the provisions of Section 112, sixth paragraphof 35 U.S.C. are not to be invoked unless the specific terms “means for”or “step for” are recited in a claim.

1. A magnetoresistive sensor element comprising: a magnetoresistivestrip comprising a plurality of serial segments, adjacent ones of thesegments having different tilt angles.
 2. The magnetoresistive sensorelement of claim 1, wherein a width of at least one of the plurality ofserial segments varies from a width of at least one other of theplurality of serial segments.
 3. The magnetoresistive sensor element ofclaim 1, wherein the magnetoresistive strip is one of a giantmagnetoresistive (GMR) strip, a tunneling magnetoresistive strip (TMR)or an anisotropic magnetoresistive strip (AMR).
 4. The magnetoresistivesensor element of claim 1, wherein the different tilt angles comprisepositive and negative tilt angles.
 5. The magnetoresistive sensorelement of claim 1, wherein the different tilt angles comprise differentdegrees of tilt.
 6. The magnetoresistive sensor element of claim 1,wherein the magnetoresistive strip comprises a center segment having atilt angle of 0 degrees.
 7. The magnetoresistive sensor element of claim1, wherein a first plurality of serial segments form a first substripand a second plurality of serial segments comprise a second substrip,the first and second substrips being serially connected.
 8. Themagnetoresistive sensor element of claim 7, wherein the first and secondsubstrips are arranged substantially parallel to one another.
 9. Themagnetoresistive sensor element of claim 1, wherein a first plurality ofserial segments form a first substrip and a second plurality of serialsegments comprise a second substrip, the first and second substripsbeing connected in parallel.
 10. The magnetoresistive sensor element ofclaim 1, wherein a first plurality of serial segments form a firstsubstrip and a second plurality of serial segments comprise a secondsubstrip, the first and second substrips having different widths. 11.The magnetoresistive sensor element of claim 1, wherein the adjacentserial segments are connected by a connector.
 12. A magnetoresistivesensor element comprising: a magnetoresistive strip comprising at leasttwo segments serially coupled, a first segment having a negative curveand a second segment having a positive curve.
 13. The magnetoresistivesensor element of claim 12, wherein the magnetoresistive strip is one ofa giant magnetoresistive (GMR) strip, a tunneling magnetoresistive strip(TMR) or an anisotropic magnetoresistive strip (AMR).
 14. Themagnetoresistive sensor element of claim 12, wherein themagnetoresistive strip is generally S-shaped.
 15. The magnetoresistivesensor element of claim 12, wherein the magnetoresistive strip comprisesa third segment having a 0-degree tilt angle and coupling the firstsegment with the second segment.
 16. The magnetoresistive sensor elementof claim 12, wherein the first and second segments are arranged opposingeach other in a circular configuration with a first end of the firstsegment coupled to a first end of the second segment and the second endsof each of the first and second segments being uncoupled.
 17. Themagnetoresistive sensor element of claim 16, wherein the first ends ofthe first and second segments are coupled by a connector.
 18. Amagnetoresistive sensor element comprising: a magnetoresistive stripcomprising a plurality of serially coupled segments, adjacent ones ofthe plurality of serially coupled segments having different widths. 19.A method comprising: forming a magnetoresistive strip having a first endand a second end; and adjusting a width of the first and second endsrelative to a width of a center of the magnetoresistive strip tocompensate for an unwanted magnetic field component.
 20. The method ofclaim 19, wherein adjusting comprises making the width of the first andsecond ends smaller than the width of the center.
 21. The method ofclaim 20, wherein adjusting comprises making the width of the first andsecond ends smaller as the unwanted By magnetic field componentincreases.
 22. The method of claim 19, wherein adjusting comprisesmaking the width of the first and second ends larger than the width ofthe center.
 23. The method of claim 22, wherein adjusting comprisesmaking the width of the first and second ends larger as the unwanted Bymagnetic field component decreases.
 24. The method of claim 19, whereinforming comprises serially coupling a plurality of magnetoresistivestrip segments to form the magnetoresistive strip.
 25. The method ofclaim 19, wherein forming comprises forming adjacent ones of theplurality of magnetoresistive strip segments with different widths.